An electron gun is a device which emits free electrons, shapes and accelerates them to form an electron beam. The physics design of an electron gun is usually done with the aid of modern computer codes which will predict the size and divergence of the beam for a given geometry and current. Examples of several different types of electron guns are the Rogowski gun, the telefocus gun and the Pierce gun.
Hot cathodes are the most frequently used emitters although many other materials emit free electrons. Several types of hot cathodes are commonly used in electron gun designs, such as tantalum wire or disc emitters, tungsten wire or disc emitters, thoriated carburized tungsten wire emitters, oxide cathodes, dispenser cathodes and lanthanum hexaboride (LaB.sub.6) cathodes.
Some of the above types of cathodes in commercial use involve mounting the electron emissive material on a metal tube cathode which surrounds a cathode heater and which is secured by means of an insulating support disc to a surrounding control grid. See for example, U.S. Pat. Nos. 3,244,927 and 3,826,947. Neither of these patents disclose exactly how the electron emissive material is secured to the cathode tube but typically this would be done by brazing. U.S. Pat. No. 4,215,457 is directed to a specific technique for securing the electron emissive material to the cathode tube and according to one embodiment described this involves pressing a porous sintered molding of refractory metal impregnated with electron emissive material into a metal foil holder which partly overlaps a free surface of the molding. The holder is then welded to the tube.
The technique disclosed in U.S. Pat. No. 4,215,457 cannot be used for securing LaB.sub.6 as the electron emissive material nor can any of the commercially available techniques. This is because LaB.sub.6 is extremely reactive with the refractory metals, such as the foil of U.S. Pat. No. 4,215,457, normally used in securing electron emissive materials.
The present invention is particularly concerned with securing LaB.sub.6 to a cathode tube of an electron gun. This material is much less sensitive to gas bursts and oil or metal vapours which can destroy in a short time active cathodes such as dispenser cathodes or oxide coated cathodes. Cathodes using LaB.sub.6 material have been used in research laboratories, one common technique involving mounting an LaB.sub.6 disc by press fit in a graphite mount and securing the graphite mount to a surrounding stainless steel ring using radially extending tungsten wires. This technique requires delicate fabrication steps and results in a device which is not rugged. In particular, the LaB.sub.6 disc could become loose in its graphite mount resulting in a decrease in thermal conductivity.